The invention relates to electrically operated conveyors in general, and more particularly to improvements in electric monorail conveyors of the type wherein the electric motor or motors of one or more vehicles contact a third rail or live rail and the vehicle or vehicles are confined to travel along one or more guide members, e.g., along one or more overhead rails in an assembly plant for motor vehicles.
A drawback of many presently known electric conveyors is that their vehicles cannot be arrested in accurately determined positions which are best suited for the placing of a load onto a decelerated and arrested vehicle or for removal of a portion of or the entire load from a vehicle which is about to be accelerated. The load can be removed for the purpose of assembling it with other parts in the production or assembly line, for transfer onto another conveyor or for treatment prior to assembly with other parts. It is desirable to rapidly accelerate the vehicles from zero speed to maximum speed as well as to rapidly decelerate oncoming vehicles for stoppage at an accurately determined location in an assembly plant or the like.
Vehicles which are equipped with electric motors consuming polyphase current cannot be arrested with less than a tolerance of approximately .+-.20 mm which is well beyond the acceptable range in many assembly plants. This is due to the fact that such motors operate within a certain RPM range which is not conducive to rapid and predictable braking.
It is also known to equip the vehicles (also called trolleys) of electric conveyors with changeablepole electric motors. This renders it possible to reduce the RPM of the motor while the vehicle approaches a load applying or a load receiving station. Such motors can ensure that the range of tolerances is reduced well below .+-.20 mm; however, the braking action is still unsatisfactory for a number of purposes. Furthermore, drive means employing changeable-pole motors are expensive and their control circuits are too complex and too costly for a large number of applications. The situation is aggravated if a plant employs a large number of conveyors or a conveyor system with a large number of vehicles.
Accurate positioning of vehicles at the loading, unloading and/or transfer stations can be achieved if the vehicles are equipped with d-c motors. Such motors are more sensitive than polyphase current motors and thus require extensive maintenance. Moreover, the control circuitry for such motors is complex and expensive.
It was further proposed to employ mechanical brakes which control the last stages of deceleration of electrically driven vehicles. Such systems are bulky and to expensive for many applications.